How specific sensory stimuli evoke specific behaviors is a fundamental problem in neurobiology. Most odorants elicit attraction or avoidance depending on their concentrations and identity, as well as the nature of the neural circuits they activate. Such odorants, moreover, typically activate combinations of olfactory sensory neurons (OSNs), complicating the dissection of the circuits translating odor recognition into behavior. Carbon dioxide (CO2), in contrast, elicits avoidance over a wide range of concentrations in the fly, Drosophila melanogaster, and activates only two populations of OSNs when examined by a sensitive, in vivo calcium imaging technique. Previous studies showed that OSNs expressing GR63a & GR21a receptors, the first CO2 olfactory neurons identified, is essential for avoidance to low concentrations of CO2, but it remained unclear the function of the other neurons activated by CO2. Here, we propose to determine that the putative 2nd CO2 OSNs and its cognate receptor that belongs to a member of the recently identified Ionotropic Glutamate Receptors (IRs) family are necessary and sufficient for detection of and avoidance to high CO2 concentrations and similar odorants such as acids. To address these questions, we will perform in vivo calcium imaging and behavioral assays. Determining subcellular localization of the 2nd CO2 receptor will predict whether or not the receptor directly interacts with odorants. To better understand central circuits mediating avoidance behavior, we will trace its projections into higher brain centers and compare them to the 1st CO2 pathway whether these two pathways converge upon a same target neuron in a higher brain center such as the lateral horn. Because CO2 and acids released by warm-blooded hosts are essential olfactory cues for the mosquito, and a homolog of the 2nd CO2 receptor is expressed in the mosquito antenna, we plan to examine whether the mosquito OSNs expressing the homolog are activated by CO2 and acids. PUBLIC HEALTH RELEVANCE: Relevance Insects transmit diseases to humans and animals including livestock, and cause serious threats to health and enormous losses to agricultural output. Many insects respond to their human and animal hosts primarily through carbon dioxide (CO2) and lactic acid, key olfactory cues emanating from mammals. These olfactory cues activate defined populations of olfactory sensory neurons in the mosquito that express the same odorant receptors as in Drosophila. Understanding how the Drosophila sensory receptors are activated by CO2 and acids, and the mechanism by which their neural circuits trigger behavioral responses to these stimuli would help us develop better strategies to prevent transmission of insect-born diseases by mosquitoes, tsetse flies, and other pathogenic insects.